The present invention relates to an arrangement for checking the tightness of a safety-relevant valve in an antilock braking system (ABS) for a road vehicle, and, more particularly, to an arrangement in which an electrically operable test valve is provided by means of which the pressure output of the auxiliary pressure source can be connected to the pressureless reservoir of the auxiliary pressure source to achieve a pressure drop in the pressure supply line leading to a pressure modulator.
An arrangement is shown in DE 37 23 875 A1 in connection with an antilock braking system in which the wheel brakes which can be subjected individually or jointly to an antilock braking control are connected to a volume-variable output pressure chamber of a pressure modulator such that, while the output pressure chamber is blocked off from the brake booster of the brake system, a pressure reduction in the connected wheel brake can be achieved by increasing the volume of the output pressure chamber. The output pressure chamber of the pressure modulator is pressure-tightly separated off movably by a piston from a control pressure chamber, into which the high output pressure of an auxiliary pressure source is permanently introduced via an inlet control valve. This pressure urges the modulator piston into its one end position associated with minimal volume of the output pressure chamber and is held in this position which is assigned to normal braking operation. A non-return valve, which prevents a drop in the pressure in the control pressure chamber in the event of a brief failure of the auxiliary pressure source, is connected in series with the inlet control valve. An outlet control valve permits the basic position of which is its blocking position, the control pressure chamber of the pressure modulator to be relieved with respect to the pressureless reservoir of the auxiliary pressure source, as a result of which pressure reduction phases of the antilock braking control can be controlled. The pressure modulator is provided with a prestressed restoring spring which acts on the modulator piston in the sense of a displacement of the piston, bringing about an increase in the volume of the output pressure chamber.
It is possible, however, for the modulator piston to be held in its end position corresponding to minimal volume of the output pressure chamber by the auxiliary pressure introduced into the control pressure chamber, against the restoring force of the spring and against the brake pressure which can be introduced into the output pressure chamber by the brake booster. Furthermore, a position sensor monitoring the position of the modulator piston is provided and generates electric output signals characteristic of the piston position and changes of that position.
On the basis of these output signals of the position sensor, through suitably controlled test cycles, it is possible to detect malfunctioning states of an ABS control valve designed as a solenoid valve as well as of the inlet control valve and the brake pressure control valves, possibly even to detect a sluggishness of the pressure modulator piston. No checking is, however, possible of the non-return valve which is intended to hold the pressure in the control pressure chamber of the pressure modulator in the event of a failure of the auxiliary pressure source.
Therefore, the known arrangement is disadvantageous in that, in the event of a leakage of the non-return valve and a simultaneous failure of the auxiliary pressure source, a failure of the brake circuit may occur, which failure is highly problematical for safety reasons, especially since a leakage of this safety non-return valve cannot be detected as long as the auxiliary pressure source is intact.
An object of the invention is therefore to improve an arrangement of the generally known type such that a leakage of the non-return valve connected between the pressure output of the auxiliary pressure source and the control pressure chamber of the pressure modulator can be reliably detected.
This object has been achieved according to the present invention by providing an electrically operable test valve for connecting the pressure output of the auxiliary pressure source to the pressureless reservoir of the auxiliary pressure source to achieve a pressure drop in the pressure supply line leading to the pressure modulator.
By virtue of this test valve which can be switched over, for example at regular time intervals for short test periods, into its functional position bringing about the pressure drop in the control pressure chamber, a piston displacement caused by a leakage of the safety non-return valve during a test cycle can be detected in a simple way and the output signal of the displacement sensor triggered for this can be used as a leak-indicator signal. The gain in safety achieved as a result is considerable.
Structurally simple alternative configuration of the resilient non-return element provided as part of the pressure modulator can include a prestressed coil restoring spring accommodated over part of its length by a control blind core of a modulator piston, and also a cup restoring spring supported on the face wall of the piston and on an end face wall of the modulator housing.
In another embodiment of the present invention, for two pressure modulators, each assigned to one of the wheel brakes of a brake circuit of the vehicle, only one position sensor is required on one of the pressure modulators. To this extent, a reduction in technical complexity is achieved.
A particularly simply design of the test valve as a 2/2-way solenoid valve is possible, however, whenever the auxiliary pressure source operates at a relatively low output pressure level. In this case, a particularly favorable configuration of the pressure modulator is as a pressure intensifier with a piston stage of greater diameter movably limiting the control pressure chamber, and a piston stage of a smaller diameter which movably limits the output pressure chamber of the pressure modulator.